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Pump–probe measurements can be used to obtain information on ultrafast phenomena.
The general principle is the following.
A sample (e.g. a SESAM) is hit by some pump pulse, which generates some kind of excitation (or other modification) in the sample.
After an adjustable time delay (controlled with an optical delay line), a probe pulse hits the sample, and its transmission or reflection is measured.
By monitoring the probe signal as a function of the time delay, it is possible to obtain information on the decay of the generated excitation, or on other processes initiated by the pump pulses.
The probe signal is typically averaged over many pulses, and a fast photodetector is not required.
The temporal resolution is fundamentally limited only by the pulse duration of pump and probe pulses.

Figure 1:
Reflectivity change in a semiconductor saturable absorber, hit by a short pulse at t = 0.

The wavelengths of pump and probe beam do not need to be identical.
A so-called two-color pump–probe measurement, based on two synchronized sources of short pulses (e.g. a laser and an optical parametric oscillator, or two parametric oscillators pumped with the same laser) has additional capabilities in ultrafast spectroscopy.
It is vital in such cases to ensure tight synchronization of the different laser sources with a very low relative timing jitter, as the jitter could otherwise spoil the attainable temporal resolution.

Pump–probe measurements can be used, for example, to monitor the recovery of a saturable absorber after its excitation, the speed of diffusion of photoexcited carriers, or the melting of a sample after being hit by an intense pulse.